1. Field of the Invention
This invention relates to radiation detectors and, more particularly, to the synchronous detection of infrared radiation using a plurality of photoconductive detectors.
2. Background Art
The use of photoconductive detectors for measuring radiation is well known. Because of their high sensitivity, photoconductive detectors such as PbS and PbSe are particularly effective in measuring infrared radiation. Detection of infrared radiation is used by the military for tracking warm vehicles and in night vision devices, is used by medical instrument manufacturers for measuring glucose and other body constituents in a non-invasive manner and is used by scientific instrument manufacturers for measuring chemical composition and structure.
In general, the resistance of the photoconductive detector changes when the radiation falls on its surface. Resistance changes can be measured as an electrical signal change and the intensity of the detected radiation can be estimated by the magnitude of resistance change.
Photoconductive detectors typically require a bias current or voltage to operate, such as a direct current bias. The sensitivity of the detector is proportional to the magnitude of the applied bias. It is preferred to supply a high bias to such a detector to increase its sensitivity and to overcome the noise of the electronics associated with the detector in an overall detection system.
The bias voltage applied to a photoconductive detector also causes current to flow in the absence of incident radiation. This current, referred to as the "dark current" or "dark signal", is usually large when compared to the current changes resulting from incident radiation. The detection of the small, radiation related signal, which is added to the large, dark signal, is often difficult. In addition, this dark signal is often the source of most of the noise in a detection system.
Synchronous detection is frequently used to overcome the problems associated with the dark signal. The incident radiation is modulated at some frequency, which results in a radiation related signal generated by the photoconductive detector which is also a modulated signal. The desired radiation related signal can be separated from the DC dark signal with a DC blocking element such as a capacitor. The modulation frequency can be chosen in a region where the noise from the dark signal is low. The bandwidth of the detection system can be made narrow so that the fraction of the noise in this bandwidth is small. Synchronous detection provides additional filtering of the component of the noise that is at the modulation frequency but which is at the wrong phase.
A common narrow-band synchronous AC filter used for this purpose is the lock-in amplifier. In a typical lock-in amplifier, a capacitor is first used to separate the AC signal from the combined AC and DC signal. The separated AC signal is then rectified in synchronism with an external reference from the radiation modulation source. In its most basic form, the lock-in amplifier includes a DC blocking capacitor, an AC amplifier, an inverting AC amplifier, a multiplexer and an integrator. The multiplexer selects either the amplified signal or the amplified and inverted signal, depending on the value of the modulation related reference signal. The output of the multiplexer is a synchronously rectified DC signal that is supplied to the integrator.
When using a radiation detection system including a plurality of separate photoconductive detectors, each detector will require a separate circuit to capture the radiation related signal for that detector element. In such an application where it is desired to simultaneously process the signals of many photoconductive detectors, lock-in amplification has not been used. Such an arrangement would require too many separate components for each photoconductive detector and is too expensive to implement. As discussed above, for a lock-in amplification system, each photoconductive detector requires a blocking capacitor, two amplifiers, a multiplexer and an integrator. This is a large component requirement for a compact and efficient system. The method used in the past in such a system including a plurality of photoconductive detectors has been subtraction of the dark signal followed by amplification, integration and multiplexing of the integrated signal from several detectors to one output.
It is an object of the present invention to provide a synchronous detection system for use with a plurality of separate photoconductive elements, but in an arrangement which does not require a large number of elements for operation. It is also an object of the present invention to synchronize the signal collection aspects of the detection system with the operation of the radiation modulator.